Learning Objectives
Master the embryological origin, histological structure, and clinical pathology of the Thymus. Understand the differences between the thymic cortex and medulla, recognize the significance of the “sail sign” on neonatal imaging, and identify key diseases associated with thymic dysfunction for the USMLE Step 1.
1. Anatomy and Embryology
The thymus is located in the anterosuperior mediastinum and serves as the primary site for T-cell differentiation and maturation.
| Feature | Description |
|---|---|
| Embryological Origin | Epithelium is derived from the 3rd pharyngeal pouch (endoderm); Lymphocytes are of mesodermal origin. |
| Radiographic Sign | Neonatal thymus appears “sail-shaped” on CXR; it normally involutes by age 3. |
| Lymphocyte Source | T cells = Thymus; B cells = Bone marrow. |
2. Histological Zones
The thymus is an encapsulated organ divided into a dense outer cortex and a pale inner medulla.
| Zone | Characteristics | Cell Status |
|---|---|---|
| Cortex | Densely packed with cells. | Contains immature T cells. |
| Medulla | Pale appearance; contains Hassall corpuscles (epithelial reticular cells). | Contains mature T cells. |
3. Clinical Pathologies
Thymic abnormalities are high-yield indicators of underlying immunodeficiencies or paraneoplastic syndromes.
| Condition | Clinical Presentation / Association |
|---|---|
| Absent Thymic Shadow | Seen in SCID and DiGeorge Syndrome (22q11 deletion). |
| Thymoma | Neoplasm associated with Myasthenia Gravis, SVC syndrome, Pure Red Cell Aplasia, and Good Syndrome. |
Activity:
Activity
High-Yield Clinical Pearls:
- DiGeorge Mnemonic: CATCH-22 (Cardiac defects, Abnormal facies, Thymic hypoplasia, Cleft palate, Hypocalcemia).
- Myasthenia Gravis: If a patient has ptosis and muscle weakness that worsens with use, always check for a thymoma on CT of the mediastinum.
- Hassall Corpuscles: If you see “onion-skin” like epithelial whorls in the mediastinum on histology, think Medulla of the Thymus.